North America has two migratory populations of monarch butterflies:
Eastern Population: The eastern population breeds east of the Rocky Mountains throughout much of the central and eastern U.S. and Canada. The population overwinters in the Transvolcanic Mountains of central Mexico.
Western Monarchs: The western population’s breeding grounds extend from California north to Washington and across the Sierra Nevada to Nevada and other western states. The population overwinters along the California coast, within 5 miles of the ocean. There are 200 to 300 small winter colonies scattered along the coast from Ensenada, Baja California to Marin County, California.